A black ink composition for a writing board has conventionally been prepared by dispersing a carbon black coloring agent in an organic solvent such as a ketone or a lower alcohol solvent with a dispersing agent, and by adding a release agent, a surfactant and the like.
However, the organic solvent gives a bad influence on atmosphere and a human body, and the use of the organic solvent tends to be restricted recently. Therefore, also regarding an ink composition for a writing board, an aqueous black ink composition is desired.
Examples of a conventional aqueous black ink composition for a writing board includes: the composition obtained by dispersing a pigment in an aqueous vehicle comprising an ester compound such as di(polyalkylene glycol ether) dicarboxylate and a resin composed of (partially saponified) vinyl acetate polymer, disclosed in Japanese Patent Kokai Publication 117975/1990; the composition comprising a silicone powder and a compound selected from an aqueous ester compound and a polyether modified silicone oil, in combination, as a release agent, disclosed in Japanese Patent Kokai Publication 239569/1992; the composition comprising a film forming resin, a blue pigment water dispersion and an aliphatic carboxylate emulsion, disclosed in Japanese Patent Kokai Publication 16991/1994; the composition comprising a specific phosphate compound as a release agent, disclosed in Japanese Patent Kokai Publication 184486/1994; and the composition comprising pentaerythritol fatty ester, polyglycerol fatty ester, oxyacid ester and the like as a release agent, disclosed in Japanese Patent Kokai Publication 264019/1994.
In these aqueous black ink compositions for a writing board, the vehicles and the release agents contained are those which is suitable for an aqueous pigment dispersion, by comparison with a conventional solvent-based marking ink composition for a writing board, but the pigment used is the same carbon black as that conventionally used in the solvent-based black ink composition.
Therefore, the aqueous black ink compositions for a writing board disclosed hereinabove easily cause thickening, coagulation of pigment, decreasing of writing density due to sedimentation, clogging in a penpoint, and break in writing, when they are filled in a marking pen and used repeatedly for a long period.